Pulmonary Function Tests

Overview

Requirements for Good PFT's

Normal 

Obstuctive Lung Disease 

Restrictive Lung Disease 

Upper Airway Obstruction 

Test

Requirements for Good PFT's

Obtaining useful information from pulmonary function tests requires both adequate equipment and reproducible performance. If these requirements are not met, the results must be interpreted with caution.

The American Thoracic Society (ATS) has published guidelines for the standardization of spirometry equipment and performance. Spirometric equipment should be selected to meet ATS recommendations, and at least daily monitoring and calibration must be done to obtain consistent and accurate PFT's.

Many physicians, however, are not involved in the selection of the equipment used for testing or its maintenance. But because spirometry is such an effort dependent test, they should examine each spirogram using performance criteria as set forth by the ATS. These include criteria for acceptability and reproducibility.

Criteria for acceptability include:

1. lack of artifact induced by coughing, glottic closure, or equipment problems (primarily leak)

2. satisfactory start to the test without hesitation

3. satisfactory exhalation with six seconds of smooth continuous exhalation and/or a plateau in the volume time curve of at least one second, or a reasonable duration of exhalation with a plateau

Criteria for reproducibility after obtaining three acceptable spirograms include:

1. largest FVC within 0.2L of next largest FVC

2. largest FEV1 within 0.2L of next largest FEV1

3. if the two above criteria have not been met, additional spirograms should be obtained

An acceptable spirogram should not be discarded even if it cannot be reproduced. Up to eight efforts may be performed in order to meet acceptability and reproducibility criteria. Beyond eight efforts, fatigue may play a role in the results, and additional efforts are not warranted. When reporting the actual values obtained from spirometry, the highest FEV1 and FVC should be reported and may be from different efforts. By contrast, FEF25-75, flow volume, and volume time curves should be reported from the best test curve. This is defined as the acceptable curve with the largest sum of FVC and FEV1.

Ensuring adequate patient effort depends on the technician measuring spirometry. The patient should be well coached and instructed throughout the test while the technician evaluates both the patient's effort and the spirograms. Meeting reproducibility criteria helps to ensure adequate effort because maximal patient effort leads to more reproducible results.

When interpreting PFT's, it is useful to keep the ATS guidelines in mind. Frequently not all the criteria are met, and this may limit the reliability of the test if the results are abnormal. If the spirometric results are normal, the test can be interpreted as normal even though all criteria may not be met.

Test acceptability is best determined by examining the flow volume loop and volume time curve. Variable effort, cough, and early glottic closure can be seen on the graphs but may not be apparent by simply looking at values for FEV1 and FVC.

 

Normal	Variable Effort
Early Glottic Closure	Cough

Variable effort can be detected by a flow volume loop that fails to demonstrate the normal early peak, showing that the patient failed to expire maximally when instructed to do so. Early glottic closure is seen as an abrupt cessation of flow during expiration, visible as a sharp downslope on the expiratory flow volume curve. Coughing during spirometry appears as sudden sharp spikes in the decreasing limb of the flow volume curve.

By examining the flow volume loops, the quality of a spirogram can be assessed. In addition, the technician working with the patient should comment on the patient's effort and the session as a whole. At least three efforts must be attempted to meet reproducibility criteria. 

Normal 

Spirometry should be interpreted using the flow volume and volume time curves as well as the absolute values for flows and volumes. The flow volume loop and volume time curve are often overlooked but provide valuable information. Certain disease states have characteristically shaped loops, so it is important to be able to recognize the different patterns. 
Normal values for FEV1 and FVC are based on population studies and vary according to race, height, age, and gender. They are expressed in both absolute numbers and percent predicted of normal. Some authors have suggested that defining normal by 95% confidence intervals would be more statistically appropriate, particularly at the extremes of age. Thus, a value below the 5th percentile is defined as "below the lower limit of normal." However, many laboratory and computer software programs continue to express results as percentages of predicted normal values. A physician's interpretative strategy should be adaptable to either reporting system. 

Values for FVC and FEV1 that are over 80% of predicted are defined as within the normal range. The FEV1/FVC ratio is expressed as a percentage, and a normal young individual is able to forcibly expire at least 80% of his/her vital capacity in one second. A ratio under 70% suggests underlying obstructive physiology; however, the FEV1/FVC ratio declines as a normal sequelae of aging. Thus, at advanced ages, pathologic airways obstruction is diagnosed based upon deviation from predicted FEV1/FVC values, with values below the 5th percentile best selecting patients with obstructive defects. 

Normal Flow Volume Loop 





A normal flow volume loop has a rapid peak expiratory flow rate with a gradual decline in flow back to zero. The inspiratory portion of the loop is a deep curve plotted on the negative portion of the flow axis. Inspiratory data is often overlooked, but the flow volume loop gives the opportunity of assessing this information as well. 

Normal Volume Time Curve 





The normal volume time curve has a rapid upslope and approaches a plateau soon after exhalation. The maximum volume attained represents the forced vital capacity (FVC), while the volume attained after one second represents the forced expiratory volume (FEV1). 

Obstructive Lung Disease

The primary abnormality detected by spirometry is airways obstruction. In obstructive lung diseases such as emphysema or chronic bronchitis, the FEV1 is reduced disproportionately more than the FVC resulting in an FEV1/FVC ratio less than 70 - 80%. This reduced ratio is the primary criteria for diagnosing obstructive lung disease by spirometry. At our institution, we use the following scale to grade the severity of obstruction:

 

FEV1	> 80% predicted	normal
	65 - 80%	mild
	50 - 65%	moderate
	< 50%	severe

As the obstruction becomes more severe and end expiratory air trapping develops, the forced vital capacity may be reduced as well as the FEV1; however there should continue to be a disproportionate reduction in FEV1 as evidenced by the FEV1/FVC ratio.

Example of spirometry results demonstrating mild obstruction:

 

	Meas	Pred	%Pred
FVC	2.63	3.11	84
FEV1	1.58	2.28	69
FEV1/FVC	60	73
FEF25-75	0.59	2.56	23
PEF	4.90	5.78	85

 

Mild Obstruction Flow Volume	Mild Obstruction Volume Time Curve
Severe Obstruction Flow Volume

Obstructive lung disease also changes the appearance of the flow volume curve. As with a normal curve, there is a rapid peak expiratory flow, but the curve descends more quickly than normal and takes on a concave shape, reflected by a marked decrease in the FEF25-75. With more severe disease, the peak becomes sharper and the expiratory flow rate drops precipitously. This results from dynamic airway collapse which occurs as diseased conducting airways are more readily compressed during forced expiratory efforts. On the volume time curve, this is seen as a slower ascent to maximum volume, with a gradual upsloping versus the rapid rate seen in normal individuals. This equates with a prolonged forced expiratory time demonstrable on physical exam.

The ATS recommends caution in diagnosing obstruction when a patient has a reduced FEV1/FVC ratio but normal FEV1 and FVC. As mentioned above, there is a normal age-related decline in the FEV1/FVC ratio, so normal elderly patients without airway obstruction will have a ratio below 70-80%. In this case, values below the predicted FEV1/FVC ratio aid in diagnosing obstruction. The mid-range flows (FEF25-75) are always reduced in obstructive airways disease. However, some patients have normal spirometry with the exception of a reduced FEF25-75. While normal values for FEF25-75 have broader ranges than the other spirometirc values, a mid-range flow less than 50% is likely to be abnormal. This is suggestive of possible small airways dysfunction and potentially early obstruction, but it should not be interpreted as meeting obstructive criteria. In the appropriate clinical setting, one may consider a trial of bronchodilators, bronchoprovocative testing to exclude asthma, or interpret this observation as a possible early indicator of smoking related lung disease. 

Restrictive Lung Disease 

 

	Meas	Pred	%Pred
FVC	0.96	2.75	35
FEV1	0.94	1.90	49
FEV1/FVC	98	69
FEF25-75	2.25	2.11	107
PEF	2.98	5.40	55

 

Restriction Flow Volume

Restriction Volume Time

The shape of the flow volume loop is relatively unaffected in restrictive disease, but the overall size of the curve will appear smaller when compared to normals on the same scale. Similarly, there will be a rapid upslope on the volume time curve, but such patients will reach a smaller vital capacity.

It is important to realize, however, that restrictive lung disease cannot be diagnosed by spirometry alone. With severe airways obstruction the lung volume remaining in the thorax after expiration (the functional residual capacity) may be increased to such a degree that it limits inspiratory capacity and, hence, reduces vital capacity. A reduced FEV1 and FVC are therefore only suggestive of true restrictive disease, but it is necessary to measure lung volumes to accurately diagnose restrictive physiology. 

Upper Airway Obstruction

In a variable upper airway obstruction, airflow is compromised by dynamic changes in airway caliber. During normal inspiration, airways within the thorax tend to dilate as the lung inflates while airways outside of the thorax tend to collapse due to the drop in intraluminal pressure. During expiration, the reverse is true as airways within the thorax collapse but airways outside the thorax are held open by expiratory flow.

 

Variable Extrathoracic Upper Airway Obstruction

Fixed Upper Airway Obstruction

As a result, a variable extrathoracic obstruction primarily affects the inspiratory portion of the flow volume loop, viewed as a flattening of the usual deep inspiratory curve. The expiratory portion of the loop appears relatively normal. Conversely, a variable intrathoracic obstruction mainly affects the expiratory limb, again giving a flattened appearance to that aspect of the loop. This can be difficult to distinguish from the more common small to medium sized airways obstruction that characterizes bronchitis, asthma, and emphysema. Finally, a fixed intrathoracic or extrathoracic obstruction affects both inspiration and expiration, giving a flow volume loop that has an overall box-like shape as both inspiratory and expiratory limbs flatten.

Variable extrathoracic obstructions may be caused by vocal cord paralysis, thyromegaly, tracheomalacia, or neoplasm while large airways variable intrathoracic obstructions can also result from tracheomalacia or neoplasm. Examples of fixed obstruction include tracheal stenosis, foreign body, or neoplasm.

In summary, spirometry is a valuable tool for the assessment of lung disease. By ensuring proper calibration of equipment and performance of test maneuvers, one can differentiate among several different disease processes.